Fix infinite loop in dilation tool

Summary:
KhanUtil.toFraction was dying from numerical error
built up over multiple dilations. This patch gives
a larger, safer (in terms of number of iterations)
tolerance to the KhanUtil.toFraction used to render
dilation text.

Long term I should figure out how to stabilize some of the
numerical error, which is probably caused from applying the
transformation every time the mouse moves rather than when the
mouse is released, and which could be alleviated by recomputing
things once the mouse is released.

Test Plan:
Make a really really large dilation created by 5+ maximum size
dilations repeated (previously this could crash on 3 or 4 max
size dilations):
 * Dilation of scale 41730642091001070 about (6,6)

Auditors: eater